Boolean Modifier Problem
Without seeing exactly what you were trying to do, it is hard to know how to advise you. But some general advice about using Booleans, pre version 2.91:

Both shapes should be closed volumes, with all normals pointing outwards
The shapes should not have self-intersections
The shapes should not intersect each other in such a way that edges, faces, or vertices of one shape exactly overlap edges, faces, or vertices of the other.

If any of these things is not true, Boolean might work but it is not guaranteed. This is probably why sometimes you tried, it worked,and other times it did not.
Blender 2.91 (to be released in late November) has a new Boolean solver that does not have problems with the second two of the above points. You can try it now in the daily alpha release builds of 2.91.